“We’re painfully aware that we’re going into a holiday weekend and that this is an inconvenience for people,” Portland City Commissioner Nick Fish said.

“We regret that, but we’re also guided by good science and regulations.”

The boil notice produced the expected public response: The city website crashed from heavy traffic, and bottled water flew from supermarket shelves.

Matt Smith, who works at a law firm, went to a downtown Safeway store to stock up on water for a host of afternoon meetings.

He found just a few bottles left.

“We can’t cancel them because that would cost a lot of money, and we have to keep people hydrated,” Smith said.

“But clearly I got here too late.”

The samples that tested positive for bacteria were collected this week from two uncovered reservoirs at Mount Tabor.

The Water Bureau said it collects about 240 bacterial samples per month throughout the system, and the test to determine the presence of bacteria takes 18 hours.

“The chance of any health problems related to this water test result is low,” Tri-County Health Officer Dr. Paul Lewis said.

“If any problems occur, we would expect diarrhea.”

The water bureau is investigating the cause of the contamination.

The city said contamination can occur when there is a loss of water pressure, a pipe breaks or when conditions expose drinking water to outside elements, such as animal waste.

The Water Bureau drew national attention last month when it decided not to pipe 35 million gallons of water to its customers because a teenager allegedly urinated into a reservoir at Mount Tabor. That reservoir was one of the two that tested positive for E. coli.

The reservoir is one of five the city is in the process of replacing with underground storage to comply with federal regulations.